A typical linear type voltage regulator for a battery-powered portable device should exhibit very fast response to load transients, low voltage drop, high rejection to the supply line, and above all, low current consumption so that the battery charge can be made to last longer. Current regulators are typically implemented using an n-channel MOS power transistor. The reason for preferring an n-channel transistor is that, for a given performance level, it allows the occupation of silicon area to be optimized and the value of the output capacitor to be reduced by at least one order of magnitude.
An exemplary application of a conventional type of voltage regulator is illustrated in FIG. 1. A regulator of the low-drop type having an n-channel topology, such as that shown in FIG. 1, requires a driver circuit OP1 being supplied a higher voltage, VCP, than the supply voltage, VBAT. This is a feature which has been achieved in state-of-art regulators by using a charge pump circuit 2.
The operation of the device in the circuit of FIG. 1 and its application will now be described in detail. The current consumption of the regulator can be calculated from the current Ires flowing through the divider R1-R2, plus the current draw Iop of the driver circuit OP1 for the power transistor M1.
Since the charge pump circuit 2 used for powering the driver circuit OP1 is a multiplier-by-n of the input voltage VBAT, its current draw from the battery is n times the current Iop that it delivers to the driver circuit OP1. Considering, moreover, the efficiency Eff of the charge pump circuit, the overall battery current consumption of the regulator is: EQU IREG=n/Eff*Iop+Ires.
The compensation usually employed for a regulator with this topology is of the pole-zero type, where the internal zero is to cancel out the pole introduced by the load capacitor. The outcome of such compensation is that a dominant pole is obtained, which considerably slows the response to load transients and produces a large output voltage variation.
A prior approach to this problem included increasing the bias current Iop of the differential stage of the driver circuit OP1, with the consequence of increasing the overall consumption of the regulator. This approach conflicts, however, with the main desirable characteristic of battery-powered devices, that is, to keep current consumption as low as possible.